In measuring the rotational speed of a shaft as, for example, an engine shaft, it is desirable to provide a digital display which indicates numerically the speed of the shaft in revolutions per minute, for example. To obtain such a digital readout of the shaft speed, pulses generated as a result of the rotation of the shaft, are counted over a predetermined time interval. The larger the number of pulses counted within that time interval, the higher is the speed of the shaft. Thus, the count of pulses within the time interval is proportional to the shaft speed.
If, however, the time interval during which the pulses are counted, is begun at an arbitrary instant in relation to the train of pulses generated from the rotating shaft, then an unstable display will result in the indication of the speed. This unstable condition is due to the possibility that within successive time intervals, the pulse count may vary by one pulse, for example, depending on the instant at which the time interval was begun in relation to the pulse train. When the digital readout has relatively few places of figures, such as two places, for example, the unstable condition is very disturbing to an observer since the last or lowest place of the figures in the digital readout will continuously change.
Accordingly, it is an object of the present invention to provide a digital tachometer which has a stable readout even when relatively few digits of places are used for indicating numerically relatively few digits or places are used for indicating numerically the speed of the shaft being measured.
It is another object of the present invention to provide a digital tachometer of the foregoing character, which is simple in construction and may be fabricated at substantially low cost.
A further object of the present invention is to provide a digital tachometer, as described, which may be economically maintained in service, and which has a substantially long operating life.